The U.S. Food and Drug Administration approved Merck & Co. Inc.'s Keytruda immune-boosting therapy to treat certain patients with a rare type of blood cancer.

Keytruda gained the regulator's backing under the FDA's accelerated approval regulations to treat adults and children with primary mediastinal large B-cell lymphoma, or PMBCL, whose disease is unresponsive to treatment or has returned after two or more prior lines of therapy.

PMBCL is a rare type of non-Hodgkin lymphoma that mainly affects young adults and is more common in women. The disease develops when the body makes abnormal white blood cells called B lymphocytes, which help protect the body against bacteria or viruses by making proteins called antibodies, and these cells build up between the lungs and behind the breastbone in an area called the mediastinum.

Merck's Keytruda boosts the immune system's ability to kill cancer cells by blocking healthy cells' PD-1 protein from linking up with a partner arm on the surface of cancer cells known as the PD-L1 protein.

Kenilworth, New Jersey-based Merck said Keytruda is the first anti-PD-1 therapy to be approved for treating PMBCL, and this authorization marks the second approved usage for the drug to treat a type of blood cancer. Keytruda is also approved to treat adults and children with classical Hodgkin lymphoma that does not respond to treatment or has returned after three or more prior lines of therapy.

Merck, one of the world's largest pharmaceutical companies by revenue, said the agency's approval was based on data from a clinical study, named Keynote-170, in 53 patients with the disease. Data from the study showed that Keytruda reduced the cancer in 45% of patients, with 11% seeing no detectable levels of the disease and 34% experiencing a reduction in the cancer without it completely going away.

Keytruda's authorization to treat PMBCL marks the second U.S. regulatory win for the drug in 2018, after it was approved by the FDA on June 12 to treat cervical cancer patients whose disease has returned or has spread to other parts of the body on or after chemotherapy and whose disease contains the PD-L1 protein.

The medicine has now been approved for multiple types of cancers, including skin cancer, lung cancer, head and neck cancer, classical Hodgkin lymphoma, bladder cancer, cancers in the urinary system and stomach cancer.